MySQL Composite Index

A composite index is an index that is used on multiple columns. It is also known as a multiplecolumn index.

Features

Let us see the features −

• MySQL allows the user to create a composite index which can consist of up to 16 columns.

• The query optimizer uses the composite indexes for queries which will test all columns in the index.

• It can also be used for queries which will test the first columns, the first two columns, and so on.

• If the columns are specified in the right order in the index definition, a single composite index can be used that would speed up certain kinds of queries on the same table.

Creating Composite Index

Let us see how a composite index can be created, during the creation of a table. It can be done using the below statement −

CREATE TABLE table_name (
   c1 data_type PRIMARY KEY,
   c2 data_type,
   c3 data_type,
   c4 data_type,
   INDEX index_name (c2,c3,c4)
);

In the above statement, the composite index consists of three columns c2, c3, and c4.

A composite index can also be added into an existing table using the ‘CREATE INDEX’ statement. Let us see how this can be done

Query

CREATE INDEX index_name
ON table_name(c2,c3,c4);

If there is a composite index on (c1,c2,c3), then the user would have indexed search capabilities on one the below mentioned column combinations −

(c1)
(c1,c2)
(c1,c2,c3)